1. Field of the Invention
The present invention relates to an insulating ceramic for use in, for example, multilayer circuit boards. Specifically, the present invention relates to a high-frequency insulating ceramic which is advantageously used in, for example, composite multilayer circuit boards equipped with semiconductor devices and various electronic parts and which can be obtained by firing in conjunction with conductive materials such as copper and silver, as well as to a multilayer ceramic substrate, a ceramic electronic part, and a laminated ceramic electronic part each using the insulating ceramic.
2. Description of the Related Art
Recent tendencies to accelerate the use of electronic equipment in higher frequencies keep on expanding. With such demands for the accelerating, higher-density mounting and higher-density packing of electronic parts which are used in such electronic equipment are still increasing. To satisfy these demands, multilayer circuit boards are conventionally used as substrates on which semiconductor devices and various electronic parts are mounted. In such a multilayer circuit board, the substrate houses a conductor circuit and an electronic part functional device to thereby further miniaturize electronic equipment.
Alumina has been conventionally frequently used as a material for constituting the multilayer circuit board.
Alumina has a relatively high firing temperature of 1500xc2x0 C. to 1600xc2x0 C., and refractory metals such as Mo, Moxe2x80x94Mn, and W must be generally used as materials for conductive circuits housed in such a multilayer circuit board composed of alumina.
However, these refractory metals have a high electric resistance. Strong demands have been therefore made for the use of a metal such as copper as a conductive material, which metal has a lower electric resistance and is available at a lower cost than the refractory metals. To use copper as a conductive material, the use of a glass ceramic or crystallized glass which can be obtained by firing at low temperatures of 1000xc2x0 C. or less has been proposed (e.g., Japanese Unexamined Patent Application Publication No. 5-238774).
However, such known substrate materials which can be obtained by firing at low temperatures have a low mechanical strength and a low Q-value, and the firing process tends to affect the type and proportion of deposited crystal phases of such materials.
Accordingly, it is an object of the present invention to provide an insulating ceramic which can solve the problems of the conventional technologies, can be obtained by firing at low temperatures, can be obtained by firing in conjunction with conductive materials having relatively low melting points such as silver and copper, has satisfactory mechanical strength and a high Q-value, and is insensitive to the type and proportion of deposited crystal phases.
Another object of the present invention is to provide a multilayer ceramic substrate, a ceramic electronic part and a laminated ceramic electronic part, each of which is composed of the insulating ceramic, which has satisfactory mechanical strength and a high Q-value, and is insensitive to the type and proportion of deposited crystal phases.
After intensive investigations to solve the above problems, the present inventors found that the deposition of MgAl2O4 crystal phase and Mg3B2O6 crystal phase and/or Mg2B2O5 crystal phase as major crystal phases can yield a higher Q-value and a higher reliability. This is because the deposition of Mg3B2O6 crystal phase and/or Mg2B2O5 crystal phase as major crystal phases in addition to MgAl2O4 crystal phase stabilizes boron in the glass to thereby improve reliability and sinterability. The present invention has been accomplished based on these findings.
Specifically, the present invention provides, in a broad aspect, an insulating ceramic including a fired mixture of a MgOxe2x80x94MgAl2O4 ceramic and a borosilicate glass, in which MgAl2O4 crystal phase and at least one of Mg3B2O6 crystal phase and Mg2B2O5 crystal phase are deposited as major crystal phases. In this context, xe2x80x9cmajorxe2x80x9d means that of the phases present, the MgAl2O4 crystal phase and the Mg3B2O6 and/or Mg2B2O5 crystal phase are present in the greatest amounts.
The borosilicate glass for use in the present invention preferably includes boron oxide, silicon oxide, magnesium oxide and an alkali metal oxide. The combination use of MgOxe2x80x94MgAl2O4 with a glass composition including at least boron oxide (B2O3), silicon oxide (SiO2), magnesium oxide (MgO) and an alkali metal oxide (e.g., Na2O, K2O or Li2O) allows the MgAl2O4 crystal phase and Mg3B2O6 crystal phase and/or Mg2B2O5 crystal phase to deposit as major crystal phases to thereby yield a high Q-value.
In this case, the borosilicate glass preferably includes about 15 to 65% by weight of boron oxide in terms of B2O3, about 8 to 50% by weight of silicon oxide in terms of SiO2, about 10 to 45% by weight of magnesium oxide in terms of MgO and 0 to about 20% by weight of an alkali metal oxide in terms of R2O, wherein R is an alkali metal.
If the content of boron oxide in borosilicate glass is less than about 15% by weight in terms of B2O3, the ratio of boron oxide to MgO in the system is low, resulting in decreased deposition of the Mg3B2O6 crystal phase and/or Mg2B2O5 crystal phase. A high reliability and a satisfactory sinterability may not be obtained.
On the contrary, if the content of boron oxide is more than about 65% by weight, the moisture resistance of the glass may be deteriorated.
If the content of silicon oxide in the glass is less than about 8% by weight in terms of SiO2, the chemical stability of the glass may be deteriorated, and if it exceeds about 50% by weight, the resulting glass may have an increased fusing temperature or a deteriorated sinterability.
A magnesium oxide content in the glass of less than about 10% by weight in terms of MgO may retard crystallization, and a content of more than about 45% by weight may cause crystallization in the manufacture of the glass to thereby deteriorate sinterability.
The alkali metal oxide in the glass acts to decrease the fusing temperature of the glass. However, a content of the alkali metal oxide exceeding about 20% by weight may decrease Q-value.
The Mg3B2O6 or Mg2B2O5 crystal phase can be selectively deposited by appropriately adjusting the ratio of magnesium oxide to boron oxide in the system in the present invention. Specifically, the Mg3B2O6 crystal phase can be deposited when magnesium oxide (MgO) is excess such that the molar ratio of MgO to B2O3 is more than about 3:1.
To the contrary, the Mg2B2O5 crystal phase can be deposited when B2O3 is excess such that the molar ratio of MgO to B2O3 is less than about 3:1.
When the molar ratio of MgO to B2O3 is in the vicinity of 3:1, both the Mg3B2O6 and Mg2B2O5 crystal phases are deposited.
The borosilicate glass preferably further includes 0 to about 20% by weight of aluminium oxide. The addition of aluminium oxide enhances chemical stability of the glass. However, if the content of aluminium oxide exceeds about 20% by weight, a sufficient sinterability may not be obtained.
Preferably, the borosilicate glass further includes about 30% by weight or less of zinc oxide. The addition of zinc oxide (ZnO) in the above proportion decreases the fusing temperature of the glass, and the insulating ceramic can be obtained by firing at lower temperatures. A content of zinc oxide exceeding about 30% by weight may deteriorate the chemical stability of the glass.
Preferably, the borosilicate glass further includes 0 to about 10% by weight of copper oxide. The addition of copper oxide (CuO) yields the insulating ceramic by firing at lower temperatures. A content of copper oxide exceeding about 10% by weight may result in a decreased Q-value.
The weight ratio of the MgOxe2x80x94MgAl2O4 ceramic to the borosilicate glass is preferably in a range from about 20:80 to 80:20. A content of the ceramic less than about 20% by weight tends to decrease Q-value. If the content exceeds about 80% by weight, the resulting insulating ceramic may not become sufficiently dense by firing at temperatures of 900xc2x0 C. to 1000xc2x0 C.
The MgOxe2x80x94MgAl2O4 ceramic is preferably represented by xMgOxe2x80x94yMgAl2O4 where x and y are indicated by weight ratio and satisfy the following conditions: 10xe2x89xa6xxe2x89xa690; 10xe2x89xa6yxe2x89xa690; and x+y=100.
The weight percentage of MgO, x, is specified in a range from about 10 to 90. This is because x exceeding about 90 may invite a problem in moisture resistance of MgO.
If x is less than about 10, a large quantity of an expensive glass may be added for firing at temperatures of 1000xc2x0 C. or less.
In the sintered ceramic, about 5 to 80% by weight of MgAl2O4 crystal phase, and about 5 to 70% by weight of Mg3B2O6 crystal phase and/or Mg2B2O5 crystal phase are preferably deposited. Contents within the above ranges can yield high reliability, satisfactory sinterability, sufficient mechanical strength and a high Q value. A content of MgAl2O4 crystal phase less than about 5% by weight may deteriorate the strengths of the insulating ceramic. If it exceeds about 80% by weight, the resulting ceramic may not become dense by firing at temperatures of 1000xc2x0 C. or less.
If the content of the Mg3B2O6 crystal phase and/or Mg2B2O5 crystal phase is less than about 5% by weight, a reaction between magnesium oxide (MgO) and boron oxide (B2O3) may not sufficiently proceed, resulting in deteriorated sinterability and reliability and decreased Q-value. To deposit the Mg3B2O6 crystal phase and/or Mg2B2O5 crystal phase more than about 70% by weight, a large quantity of an expensive glass must be added to thereby increase cost.
In the present invention, a mixture obtained by calcining a glass composition at temperatures of about 700xc2x0 C. to 1000xc2x0 C. may be employed as the glass.
The resulting insulating ceramic according to the present invention preferably has a Q-value of 400 or more as determined at a frequency of 10 GHz. If the insulating ceramic has a Q-value of 400 or more at 10 GHz, the ceramic can be advantageously employed in circuit boards for use at high frequencies, for example at frequencies of 1 GHz or more.
In another aspect, the present invention provides a multilayer ceramic substrate which includes a ceramic plate including an insulating ceramic layer composed of the insulating ceramic, and a plurality of inner electrodes formed in the insulating ceramic layer of the ceramic plate.
In the invented multilayer ceramic substrate, a second ceramic layer having a higher dielectric constant than the insulating ceramic layer may be laminated on at least one side of the insulating ceramic layer.
The plurality, of inner electrodes in the invented multilayer ceramic substrate may be laminated via at least part of the insulating ceramic layer to thereby constitute a laminated capacitor.
Preferably, the plurality of inner electrodes include capacitor inner electrodes and coil conductors, and the capacitor inner electrodes being laminated with each other via at least part of the insulating ceramic layer, and the coil conductors being connected to each other to thereby constitute a laminated inductor.
The present invention provides, in a further aspect, a ceramic electronic part which includes the multilayer ceramic substrate and at least one electronic part device which is mounted on the multilayer ceramic substrate and constitutes a circuit with the plurality of inner electrodes.
Specifically, the invented ceramic electronic part may further include a cap fixed on the multilayer ceramic substrate so as to surround the electronic part device. A conductive cap is preferably used as the cap.
The invented ceramic electronic part preferably further includes a plurality of outer electrodes only formed on the underside of the multilayer ceramic substrate, and a plurality of through-hole conductors which are electrically connected to the outer electrode and electrically connected to an inner electrode or the electronic part device.
In yet another aspect, the present invention provides a laminated ceramic electronic part which includes a sintered ceramic composed of the insulating ceramic, a plurality of inner electrodes arrayed inside the sintered ceramic, and a plurality of outer electrodes which is formed on an outer surface of the sintered ceramic and electrically connected to any of the inner electrodes.
The plurality of inner electrodes may be arrayed so as to overlap with each other via a ceramic layer to thereby constitute a capacitor unit in a specific embodiment of the invented laminated ceramic electronic part.
Preferably, the plurality of inner electrodes in the invented laminated ceramic electronic part further includes a plurality of coil conductors connected to each other to thereby constitute a laminated inductor unit, in addition to the inner electrodes constituting the capacitor unit.